4' x 4' x 8" Router Build

[JonnyFive]

Understood. They were originally 50x50x3 like the rest of the frame but I changed them to increase the second moment of area. I can change them back easy enough.

[GrahamWhite]

I bought my rack while the parts where on their way from USA, noobie error on my part! lol
I only matched up the 20 degree angle not the DP
End up buying new 60tooth gears with a hub on and new spur gears for mod 1.0 rack and drilled out and reassembled the drives, at least now if I ever need to replace the spur gear they're only £5 :)

[Neale]

Good luck - it's a fun process!

I built something roughly similar a year or two back - here. Probably more useful if you look at the "what I would do different next time" comments!

Joe's CNC guys go rack and pinion because it is a heavily US-based community. R&P can be bought from US manufacturers who don't seem to make ballscrews which generally come from China. Not a popular choice for US builders, unlike UK/Europe. Seems to be almost a fashion thing rather than technical choice. Both can work but unless you really need R&P, ballscrews are much easier.

[JonnyFive]

Thanks for your input Neale, just read your thread - great work on documenting everything so thoroughly!! It'd be nice to see some photos of the entire machine, looks like I'm heading along a similar path to you.

Interesting what you say about the R&P. My understanding of the ballscrew / R&P decision was that although ballscrews are more accurate & efficient anything over 1m should use R&P because the ballscrew whipping limits performance - is that correct? There doesn't seem to be much of a cost difference between the two?

[Neale]

Interesting what you say about the R&P. My understanding of the ballscrew / R&P decision was that although ballscrews are more accurate & efficient anything over 1m should use R&P because the ballscrew whipping limits performance - is that correct? There doesn't seem to be much of a cost difference between the two?

This is where we get into real design decisions! I'm using 2005 ballscrews. There are a few online critical speed calculators available; the general consensus for 1750mm, fixed/floating bearing, screws is around 900-1000 rpm. That gives me 5000mm/min rapids, which is slower than ideal but which works for me using a hobby machine. In retrospect, I could have used 2010 and run faster. However, I am also using 2x3Nm motors to drive these. Pulley drive, but 1-1 gearing. This is because the motors are just about reaching their corner speed (where torque starts dropping rapidly) at about the "whip" speed of the ballscrews. I'm not entirely sure (because I haven't studied other people's designs) if the motors would handle the load of my heavy gantry using 2010 as there is also the acceleration/speed trade-off. I doa fair bit of small fiddly machining where acceleration is more important in overall cutting time than speed.

All I'm really saying is that you have to balance up all the conflicting requirements for an engineering compromise that is right for you and your intended use. My design/build might not be optimal, even for me, but at least it works! There are plenty of posts one this forum with other designs successfully using ballscrews of this kind of length and I would happily do the same again.

[JonnyFive]

That's good to know, I appreciate your input. I'll keep trawling through build logs and see what others have come up with.

Given what you have said do you think it would be better to have used torquier motors and gear them down to get the speed up, perhaps needing to go up in screw diameter as a consequence? I have no real concept of how fast is too fast / slow or how much torque is needed to cut at reasonable feeds. I have a combination of small fiddly and large uncomplicated projects that I'd like to be able to do on my machine. Initially I'm not that concerned about speed but at some point I would like to make some money out of this and so higher speed would be nice.

[JonnyFive]

Does this look about the right amount of clearance with the z-axis at its lowest?



My plan is to have two pieces of wood for the bed, the top one being sacrificial. I've assumed the shortest cutter would stick out about 25mm and allowed this much clearance from the top of the first spoil board. This should allow for the top board to be resurfaced a few times and still give enough travel on the Z. Am I about right with this?

Cheers

[JonnyFive]

I bought my rack while the parts where on their way from USA, noobie error on my part! lol
I only matched up the 20 degree angle not the DP
End up buying new 60tooth gears with a hub on and new spur gears for mod 1.0 rack and drilled out and reassembled the drives, at least now if I ever need to replace the spur gear they're only £5 :)

Sounds like it might have been a costly way of doing things? I had considered designing my own version as there's nothing clever about it really but thought the costs would be more than buying from CNCRP.

Out of interest where did you get your aluminium extrusion from please?